1. Field of the Invention
The present invention relates to a liquid crystal display apparatus of the lateral direction electric field drive type or the twisted nematic type and in particular to a liquid crystal display apparatus having a high display quality and little residual image.
2. Description of the Related Art
Recently, a study has been made on a liquid display apparatus of IPS (in plane switching) type (or lateral direction electric field drive type) in which the molecule axis direction of oriented molecules is rotated in the horizontal direction with respect to a substrate.
The liquid crystal display apparatus of the IPS type is characterized in that it is possible to obtain a wide FOV (field of view) angle as compared to a liquid crystal display apparatus of TN (twist nematic) type.
FIG. 24 is a cross sectional view of a TFT element of a liquid crystal display (LCD) apparatus using the conventional IPS method. In FIG. 24, the TFT is formed by a gate electrode 101, a gate insulation film 102, a semiconductor layer 103, a source electrode 104 a drain electrode 105, and an insulation film 106 layered on a transparent substrate. The semiconductor layer 103 is formed in such a way that semiconductor layer 103 protrude from the gate electrode 101.
Light incident from the back light into the back of an active matrix is shaded by the gate electrode. However, as has been described above, when the semiconductor layer protrudes from the gate electrode, the light of the back light directly collides into this protrusion of the semiconductor layer and a plenty of leak current is generated, causing a residual image.
Moreover, the liquid crystal display apparatus of the IPS type in which the liquid crystal has a low transmittance has a problem that an increase of the back light quantity causes an increase of the leak current by the outward protrusion, thereby deteriorating the residual image level.
A method to improve the residual image problem by controlling characteristics of the liquid crystal material, the orientation film, and the insulation film has been suggested, for example, in Japanese Patent Publication 7-159786. In this known technique, the residual image is reduced by lowering the resistance value of the liquid crystal. However, in this method, the number of movable ions in the liquid crystal is increased and the liquid crystal has a low resistance. Accordingly, as shown in FIG. 25, more movable ions are adsorbed into an indentation of the back channels compared in the prior art. As a result, the light leak current is increased, disabling improvement of the residual image level.
On the contrary, when the semiconductor layer is smaller than the gate electrode, the overlap of the gate electrode and the drain electrode/source electrode increases the parasitic capacity in this portion. This causes a phenomenon called field through which causes irregularities in the target potential to be maintained in the pixel electrode. This appears as flickers on the screen.
Moreover, the overlap of the drain electrode/source electrode and the semiconductor layer requires a predetermined area to assure ON current.
Moreover, the overlap of the source electrode and the semiconductor layer requires a predetermined area for a necessary ohmic contact.
Considering the aforementioned, in order to reduce the residual image, it is considered preferable to make the width of the semiconductor layer in the gate length direction identical to the gate length.
Moreover, as a method for forming the semiconductor layer properly in relation to the gate electrode, there is a method to perform a resist exposure from the back of the gate electrode. This methods includes a step of patterning of the gate electrode, a step of forming a semiconductor layer thereon, a step of forming a resist thereon, and a step of exposure from the back of the gate electrode. In this case, the resist is exposed along the gate electrode pattern. Accordingly, the semiconductor layer is formed properly in relation to the gate electrode.
Considering the aforementioned, in order to improve the residual image level, it is considered preferable to make the width of the semiconductor layer in the channel length direction identical to the gate width in the channel length direction.
Moreover, as a method for forming the semiconductor layer properly in relation to the gate electrode, there is a method to perform, after formation of the gate electrode, an exposure from the back of an active matrix substrate for forming the semiconductor substrate (back surface exposure method). This methods includes a step of patterning of the gate electrode, a step of forming a semiconductor layer thereon, a step of forming a resist thereon, and a step of exposure from the back of the active matrix substrate. In this case, the resist excluding the portion on the gate electrode pattern is exposed. Accordingly, the semiconductor layer is formed properly in relation to the gate electrode.
However, when forming the semiconductor layer properly in relation to the gate electrode by the resist exposure from the back of the gate electrode, the used for exposure from the back is absorbed by an amorphous silicon layer, disabling the exposure.
Moreover, this method has a problem that the back surface of the active matrix substrate is easily scratched and cannot be used in practice.